In 2013, the MPEG group in the International Standardization Organization (ISO), jointly with the International Telecommunications Union (ITU), released the first draft of the HEVC (also known as H.265) video coding standard. More recently, the same group has released a call for evidence to support the development of a next generation coding standard that provides improved coding performance over existing video coding technologies.
As used herein, the term ‘bit depth’ denotes the number of pixels used to represent one of the color components of an image. Traditionally, images were coded at 8-bits, per color component, per pixel (e.g., 24 bits per pixel); however, modern architectures may now support higher bit depths, such as 10 bits, 12 bits or more.
In a traditional image pipeline, captured images are quantized using a non-linear opto-electronic function (OETF), which converts linear scene light into a non-linear video signal (e.g., gamma-coded RGB or YCbCr). Then, on the receiver, before being displayed on the display, the signal is processed by an electro-optical transfer function (EOTF) which translates video signal values to output screen color values. Such non-linear functions include the traditional “gamma” curve, documented in ITU-R Rec. BT.709 and BT. 2020, and the “PQ” (perceptual quantization) curve, described in SMPTE ST 2084 and Rec. ITU-R BT. 2100.
As used herein, the term “forward reshaping” denotes a process of sample-to-sample or codeword-to-codeword mapping of a digital image from its original bit depth and original codewords distribution or representation (e.g., gamma or PQ, and the like) to an image of the same or different bit depth and a different codewords distribution or representation. Reshaping allows for improved compressibility or improved image quality at a fixed bit rate. For example, without limitation, reshaping may be applied to 10-bit or 12-bit PQ-coded HDR video to improve coding efficiency in a 10-bit video coding architecture. In a receiver, after decompressing the reshaped signal, the receiver may apply an “inverse reshaping function” to restore the signal to its original codeword distribution.
In video coding, the term “in-loop filtering” refers to filtering processes that apply to reconstructed images and affect both the quality of the video output and the quality of the reference pictures used for motion compensation. These in-loop filters are distinct from other, optional, post-processing filters that may still be applied by a decoder. For example, in HEVC, in-loop filters include a deblocking filter and the sample-adaptive offset (SAO) filter. These filters are also commonly denoted as “loop filters” (e.g., block 130 in FIG. 1A or block 150 in FIG. 1B).
As appreciated by the inventors here, as development begins for the next generation of a video coding standard, improved techniques for adaptive loop filters are desired. Methods of this invention can be applicable to a variety of video content, including, but not limited, to content in standard dynamic range (SDR) and/or high-dynamic range (HDR).
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.